The present invention relates to a method for adding solid zinc or zinc-aluminum to galvanizing baths.
Ferrous materials are widely used in building structures and other components such as fasteners and automotive parts. Since they are readily corroded, various means have been employed to protect them from corrosion. Among these means, hot dip zinc plating or galvanizing is applied to a wide variety of ferrous materials ranging from small-sized joint members such as bolts to large-sized structural members such as xe2x80x9cIxe2x80x9d-shaped steel beams to wire and sheet products such as wire coil and automobile parts.
In general, galvanizing of iron and iron-based alloys is carried out in a galvanizing tank containing a molten bath of zinc metal by either a batch or continuous process. Typically, a batch process is used to galvanize discrete parts by dipping them into the bath while a continuous process is used to galvanize wire or sheet product by passing it into and out of the bath using rollers.
A problem that arises in hot galvanizing coating processes is the formation of impurities, known as xe2x80x9cdross,xe2x80x9d on the exposed surface of and in the molten coating bath. It is desirable to minimize the extent to which the dross is capable of contacting the surface of ferrous metal and ferrous based alloy parts as they enter and exit the molten coating bath. There are various forms of dross that can be present in the molten coating bath. One type of dross is caused by the oxidization of the coating metal or alloy. Another type of dross is due to the formation of intermetallic compounds between the zinc or other metal constituent in the bath with iron that is dissolved from the surface being galvanized or that may otherwise be carried into the bath (e.g., iron fines). These compounds form insoluble particles that are denser than the molten bath and settle to the bottom of the galvanizing vessel containing the bath. As a result, an undesirable sludge forms that can be entrained in the molten metal of the coating. Both types of dross reduce the quality of the coating, a problem that is particularly deleterious in applications requiring high surface finishes, e.g., automotive sheet steel. Thus, prior art galvanizing processes have attempted to inhibit the formation of dross in or remove dross formed from molten galvanizing baths.
In conventional hot dip coating processes, this has been mechanically accomplished by employing relatively elaborate devices that circulate the dross to prevent it from accumulating at locations where the dross could undergo substantial contact with the ferrous-containing part or stock entering or exiting the molten coating bath. It is also known that a specified amount of aluminum is added to adjust the chemistries of the molten galvanizing bath. This is typically accomplished by adding zinc or zinc-aluminum alloy ingots or bars that are approximately 2 inches by 3 inches by 24 inches in dimension and weigh approximately 40-50 pounds each. The composition of these ingots is typically that provided for in ASTM B860-95 Standard Specification for Zinc Master Alloys for Use in Hot Dip Galvanizing, which specifies zinc master alloys including zinc or zinc-aluminum brighteners used in hot dip galvanizing for the purpose of adjusting the concentration of alloying elements in the molten zinc bath. Typically, 90/10 Zn/Al High Purity alloy ingots containing between about 10 to about 13 percent aluminum are used to lower the melting point of the ingots to facilitate the melting and incorporation of the zinc or zinc-aluminum alloy addition into the bath. However, this method of addition generally requires about 16 hours to make a 5000 pounds alloy addition of ingots to a molten galvanizing bath.
U.S. Pat. No. 6,426,122 discloses a method for hot dip galvanizing comprising the steps of dividing a plating vessel holding a molten metal into a plating tank and a dross removing tank; conducting hot dip galvanizing to a steel strip by immersing it in the molten metal bath; then transferring the molten metal bath from the plating tank to the dross removing tank; removing a dross from the molten metal bath in the dross removing tank; and recycling the molten metal bath from the dross removing tank to the plating tank through an opening located on the plating tank. The apparatus for galvanizing comprises a plating tank, a dross removing tank, a means to transfer the molten metal bath from the plating tank to the dross removing tank, and an opening located on the plating tank to recycle the molten metal bath from the dross removing tank to the plating tank.
U.S. Pat. No. 5,827,576 discloses a hot dip coating apparatus and method for coating a continuous steel strip, wire, or like continuous member with zinc, aluminum, tin, lead, or alloys of each. A molten coating bath is contained in a vessel having a bottom opening upwardly through which the steel member is directed. Magnetic containment devices located below the vessel""s bottom opening prevent the escape of molten metal from the vessel through the opening. The molten coating metal bath can be replenished by metal from a wire drawn from a spool of wire. The wire may be fed or directed downwardly by guide rolls through a vertically disposed induction heating coil, located directly above vessel, for heating the wire to a desired temperature, or its melting point. As the wire is fed downwardly through the heating coil, the wire is melted.
U.S. Pat. No. 5,026,433 discloses a method of producing a grain refined copper base alloy. The alloy contains iron in the amount of less than 2.3% by weight and is cast into an ingot by conventional direct chill casting. Calcium is added to the melt before casting, preferably in the form of a copper-clad or iron-clad calcium feedwire.
U.S. Pat. No. 4,512,800 discloses an apparatus for adding wire-form processing elements directly into a molten material, particularly the addition of calcium to iron and steel in the molten state. The apparatus comprises a heat resistant nozzle positionable relative to the surface of the molten material such that an inlet is disposed above the surface and an outlet is disposed beneath the surface. A mechanism for feeding the wire through the nozzle directly into the molten material and a system for injecting a substantially inert gaseous medium into the molten material together with the wire are provided. The inert gas is reported to substantially prevent closure of the nozzle by solidified molten material and promote mixture of the processing elements with the molten material through gas bubble agitation.
U.S. Pat. No. 4,481,032 discloses a process for adding calcium to a bath of molten ferrous material in which a calcium metal-containing wire is fed through a refractory lance into the bath. Recirculatory stirring of the molten ferrous material is accomplished with an inert gas flow through the lance. The calcium-containing wire is fed at such a rate that it substantially bends towards the horizontal direction after it leaves the lance and melting of the calcium in the wire occurs primarily in or directly below a region of downwelling of the molten ferrous material. Suitable wire feeding rates are reported to depend upon the disposition of the lance in the bath and the composition (e.g., clad or unclad) and cross-sectional dimensions of the calcium metal-containing wire.
U.S. Pat. No. 4,330,328 discloses a process and apparatus for producing a copper metal or alloy wherein a first material is added to a molten metal, consisting essentially of copper preferably in a furnace. After the first material is added, the molten metal is passed through a filtration device to remove particulate matter from the molten metal and/or reduce the oxygen content of the molten metal. After filtration, a second material addition preferably comprising a zirconium material is made. The zirconium material is added to the melt preferably in powder form by wire-feed apparatus. Although in the preferred embodiment the process is used to make a copper alloy, it is alleged that the process can also be used to make alloys having other base metals, i.e., aluminum.
U.S. Pat. No. 4,088,475 discloses a method for adding reactive elements to molten copper or copper base alloys. The method prevents unwanted reactions and oxidation by adding the reactive elements to the molten metal in the form of a powder mixture placed within tubing that is compatible with the molten metal. The filled tubing is sealed and drawn down, if desired, to an appropriate size for reportedly rapid melting within the molten metal and consequent rapid dissolution of the reactive elements throughout the molten metal.
U.S. Pat. No. 3,738,827 discloses a method for adding solid metal to molten metal, allegedly without deleterious reaction with air. The method is characterized by providing the solid metal in rod or wire form clad with a material compatible with the molten metal and feeding the clad material into the molten metal. The method is reportedly particularly useful in deoxidizing molten metal, especially copper base alloys with a metal of the lanthanide series.
The foregoing illustrates limitations known to exist in present galvanizing plating baths and their preparation methods. Thus it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly an alternative method for adding solid zinc or zinc-aluminum to galvanizing baths is provided including the features more fully disclosed hereinafter.
The present invention relates to a method for adding solid zinc or zinc-aluminum to galvanizing baths is provided. This method includes providing a molten mass of metal selected from the group consisting of zinc, aluminum, zinc-based alloys, or aluminum-based alloys; providing a zinc or zinc-aluminum based alloy wire; and adding the wire into the molten metal mass to enable dissolution and uniform distribution of the zinc or zinc-aluminum based alloy throughout the entire molten metal mass.